Membrane estrogen receptors (mERs) are a group of receptors which bind estrogen. Unlike the estrogen receptor (ER), a nuclear receptor which mediates its effects via genomic mechanisms, mERs are cell surface receptors which rapidly alter cell signaling via modulation of intracellular signaling cascades. Putative mERs include membrane-associated ERa (mERa) and ERß (mERß), ER-X, ERx, GPER (GPR30), and G_q-mER.